A synthetic approach that has shown promise in producing branched polyphosphonates has been the transesterification process. The transesterification process involves the reaction of a phosphonic acid diaryl ester, a bisphenol, a branching agent (tri or tetra phenol or phosphonic acid ester), and a basic catalyst carried out in the melt, usually in an autoclave. Transesterification is a chemical reaction that is an equilibrium between the starting materials and the products (polyphosphonate and phenol). The reaction is typically carried out at high temperature under reduced pressure. The by-product, phenol, is removed from the reaction by distillation; this helps shift the equilibrium toward polyphosphonate formation. One major problem with this process is that under the conditions of phenol removal, the phosphonic acid diaryl ester is also volatile and can co-distill with the phenol, leading to stoichiometric imbalance and shifting of the equilibrium which leads to lower molecular weight and reactive end groups. This problem has been addressed by the placement of a distillation column in the process that allows for separation of the phenol from the phosphonic acid diaryl diester and condensation of the diester back into the reaction vessel. This approach has only achieved limited success because some of the phosphonic acid diaryl ester is still lost during the process resulting in a stoichiometric imbalance and low molecular weight product that contains reactive end groups such as hydroxyl groups. Thus, the reaction conditions (temperature, time and pressure), stoichiometric balance of the starting materials, amount of branching agent, and amount and type of catalyst are critical parameters for synthesizing branched polyphosphonates that are free of cross-links and exhibit improved properties.
Several patents have addressed the use of branching agents (see e.g., U.S. Pat. Nos. 2,716,101; 3,326,852; 4,328,174; 4,331,614; 4,374,971; 4,415,719; 5,216,113; 5,334,692; and 4,374,971); generally, sodium phenolate is used as the catalyst in these processes, but other metal catalysts have also been used. These approaches have met with some degree of success, however, the combination of properties exhibited by these polyphosphonates are still not sufficient for general acceptance in the marketplace.
Quaternary phosphonium catalysts and aqueous-phenol based quaternary phosphonium catalyst mixtures have reportedly been used in the synthesis of polycarbonates (see e.g., U.S. Pat. Nos. 3,442,854 and. 6,291,630 B1). However, these catalysts have not been applied to the synthesis of polyphosphonates, nor is it obvious that they would work better than any other known catalyst for these materials.